There has been a great deal of interest in magnetic devices. Magnetic devices function based on the capability of generating different patterns of magnetization in a magnetizable material, in a non-volatile manner. Research has focused on exploiting giant magnetoresistance (GMR) at the nanoscale to design magnetic devices. The GMR effect has been reported in some thin-film structures composed of alternating ferromagnetic and non-magnetic conductive layers.
In an example magnetic devices, the magnetization of a cell of the magnetic device may be controlled using a magnetic field that interacts with the magnetizable material. The orientation of the magnetization can affect the resistance of portions of the magnetizable material forming the cell. Thus, for a given applied voltage, a cell with the magnetization oriented in one direction may exhibit a different resistance than if the magnetization were oriented in a different direction. As a result, the magnetizable material can be used, e.g., to store data, through changes in the magnetization direction.
Another example of a magnetic device is a magnetic tunnel junction (MTJ) device having large tunnel magneto-resistance, such as in MTJs with MgO tunnel barriers. The interest in these devices stems from the large tunnel magneto-resistance combined with their inherently non-volatile characteristics, which causes them to be considered a candidate for next generation non-volatile memory applications such as magnetic random access memory (MRAM).
In many of these proposed MTJ based magnetic memory devices, such as field switched MRAM and spin transfer torque MRAM, significant current flow is necessary to switch the magnetic free layer and therefore the state of the device. The main challenge for such devices lies in reducing the current flow necessary to manipulate the magnetization in MTJs.
Using a gate voltage to assist switching of the free layer in a MTJ could significantly lower the current necessary to switch the device state. Moreover, voltage control in MTJs would simultaneously provide compatibility with voltage based semiconductor technology. Indeed, several mechanisms have been proposed to allow voltage-assisted switching in MTJs. Those mechanisms include: electric field control of magnetic anisotropy in ferromagnetic (FM) metal/dielectric bilayers, voltage control of magnetic anisotropy in strain-coupled FM metal/ferroelectric bilayers, mechanical stress mediated magneto-electric coupling in piezoelectric/magnetostrictive bilayers, and voltage control of the exchange field in FM metal/multiferroic bilayers.
Based on those mechanisms, a number of device concepts have been proposed to reduce the switching current in MTJs. These device concepts can be separated into two categories based on the location of the gate dielectric within the MTJ stack. U.S. Publication No. 2013/0015542 A1 to Wang et al. shows and described the first device category, in which an ordered, crystalline insulator (such as magnesium oxide) serves as the gate dielectric and simultaneously acts as the tunnel barrier between a pinned magnetic layer and a free magnetic layer in a MTJ stack. (Another example MTJ structure is shown in T. Maruyama et al., Nature Nanotechnology, vol. 4, pp. 158-161 (2009).) The gate dielectric layer therefore needs to exhibit high tunneling magneto-resistance, as well as strong voltage induced effects. The dual function of the tunnel barrier therefore often results in conflicting design criteria for device optimization. U.S. Publication No. 2010/0080048 A1 to Liu et al. shows and describes the second device category, in which a dedicated layer adjacent to the magnetic free layer is used to provide the voltage functionality, separate from the tunnel barrier. The voltage-controlled layer here is made up of a piezoelectric, ferroelectric or multiferroic material. Those materials often suffer from a loss of functionality at room temperature, degradation during operation and challenging processing conditions.
Once any these device are designed and fabricated, it is difficult if not impossible to modify the functional range of such a device.